1. Field of the Invention
This invention relates to spectroscopic chemical analysis and more particularly to flow cells (sample cells) of flow-through optical detectors which are used in continuous-flow analysis, flow injection analysis, colorimetry and liquid and gas chromatography.
2. Description of the Related Art
Developments in the techniques and methodologies of chemical analysis have advanced rapidly over the last several years, especially for laboratory spectrophotometers; however, the evolution of state-of-the-art hardware designed specifically for obtaining the spectral characteristics of sample material subjected to illumination in continuous-flow analysis, FIA (flow injection analysis), colorimetry or HPLC (high performance liquid chromatography) has lagged far behind. Currently, obtaining an absorption spectrum from a continuous-flow sample system is done with complex, bulky instruments which may have inherent design limitations that increase the cost of sampling and/or reduce the performance of the detector.
In this specification, continuous-wavelength, absorption detection is the obtaining of a spectrum over a bandwidth of interest for quantitative analysis. The spectrum represents the transmittance or absorbance of the source radiation (illumination) after it has been subjected to the sample material. The bandwidth may fall within the optical range (visible spectrophotometric analysis); thus, this specification includes the area of optical detection; however, the methods and apparatus discussed herein may also apply to absorption detection in the ultraviolet (ultraviolet spectrophotometric analysis) and the infrared region (near-infrared and infrared spectrophotometric analysis) as well as the previously mentioned visible region of the spectrum. In addition, the methods and apparatus discussed herein may also apply to dual-wavelength absorption detection wherein a measuring wavelength and a reference wavelength are utilized for the absorption analysis instead of using an absorption spectrum.
A typical simple spectrophotometer (absorption detector) includes an electromagnetic radiation source, a radiation detector, a sample cell (sample chamber) and a monochromator containing a prism or grating system which dispenses the source radiation so that only a limited wavelength, or frequency, range is allowed to irradiate the sample at a time to produce an absorption spectrum for detection by the radiation (illumination) detector. In this specification the terms "illumination or radiation detector" will be used to avoid confusion with the term "absorption detector". The illumination detector converts the illumination into an electrical signal representative of the absorption spectrum. In another typical embodiment, all wavelengths of the source radiation simultaneously irradiate the sample. The absorption spectrum is dispersed such that it may either be detected by passing a limited bandwidth range of the spectrum over the radiation detector at a time or by spreading the absorption spectrum over a radiation detector acting as an array. The array detector provides an individual output signal for each limited wavelength range. Other methods are also well known to practitioners of the art for producing electromagnetic radiation (illumination) to irradiate a species and for producing an absorption spectrum signal representative of the absorption spectrum of the species which has been subjected to that illumination. Any of these methods may appropriately be applied to the teachings of this invention. However, although there are various means for producing illumination and detecting an absorption spectrum, many of these systems may not have the ability to detect absorption spectra efficiently and accurately due to inherent design limitations associated with the use of typical flow cells (sample cells).
Prior art FIGS. 1A, 1B and 1C illustrate, in simplified form, arrangements used within spectrophotometers for producing electromagnetic radiation to irradiate a sample material (species) in a continuous-flow detector-cell and for detecting an absorption spectrum. The term "detector-cell" in this specification includes a sample cell and the components associated with it. These associated components enable: (1) the source radiation to enter the sample cell to irradiate the sample, (2) the absorption spectrum to exit the sample cell for detection, and (3) the sample material (species) to enter the sample cell as a continuous-flow.
FIG. 1A is a simplified representation of a detector-cell having a continuous-flow sample cell 10a located between a means for producing radiation (optical energy) 12 and a means for producing an absorption spectrum signal 14. In this arrangement, optical energy is transmitted inline with the sample flow. The length of the optical energy path within the sample cell determines the sensitivity of the absorption detector. The longer the optical path is within the sample cell, the greater the detector's sensitivity to differences in absorption between the wavelengths or frequencies within the bandwidth of the spectrum.
In FIG. 1A, sample cell 10a is shown in a vertical-longitudinal cross-sectional view. This particular sample cell 10a is known as a "Z" flow-through channel structure. Other channel geometries are possible for, for example, FIG. 1B illustrates in another vertical-longitudinal cross-section view, another widely used flow-through sample cell 10b which has a "U" structure. The directions of flow in sample cells 10a and 10b are indicated by the arrows. The optical energy enters the sample cell in each of the FIGs. at window 16 and exits by window 18.
These flow cells (10a and 10b), shown in FIGS. 1A and 1B, are commonly used, but in order for optical radiation to be transmitted inline with the flow path of the sample material, the sample has to follow a tortuous path of flow into and out of the sample cell. One frequent problem with spectrophotometers having these types of flow cells is that they are subject to bubble noise. Often the species may have entrained gas bubbles or air bubbles. Bubble noise is caused by the bubbles becoming trapped within the sample cell due to the tortuous path of the sample through the flow cell. The natural buoyancy of the bubble in the fluid may cause it to contact and to adhere to a wall of the flow cell. When the pumping system does not provide enough flow to overcome the adherence and the friction between the bubble and the wall, it may be difficult to dislodge the bubble from the flow cell. The pumping system then causes the bubble to pulsate between pumping cycles, thus causing pulsations in the illumination intensity or causing variations in the detected absorption spectra.
One solution to the bubble problem is to use another arrangement for the sample cell as shown in the vertical cross sectional view for FIG. 1C. This sample cell 10c is a straight flow-through cell aligned vertically such that the optical energy is transmitted across the cell, i.e., transverse to the flow. However, in order to maximize sensitivity, the cell is broadened to increase the optical path length. By increasing the optical path length, the volume of the cell is increased. This increases the dead zone of the sample cell. A larger dead zone requires larger volumes of sample material for adequate separation between the samples and to prevent different species from mixing within the sample cell. In addition, this enlargement also results in reducing the total number of samples that may be processed in a continuous-flow system over a given time period.
If the optical path length is decreased to reduce the volume of the cell, the sensitivity of the detector is reduced. For example, assuming everything else remains the same, if the optical path length is halved, then the amount of absorption at various absorbing wavelengths will be substantially reduced. Sensitivity is particularly important when a heavily diluted sample which does not readily absorb at the absorbing wavelengths is used. Since so little absorption occurs across the bandwidth, then it may be difficult to determine the composition of the species especially if the sample line was subject to bubble noise or other forms of noise that is present in a plant environment.
There are other limitations with the arrangements shown in FIGS. 1A, 1B and 1C. Since the means for producing the radiation 12 and the means for detecting the absorption spectrum signal 14 are on opposite sides of the flow cell 10, these arrangements may not effectively utilize the light (radiation) entering the cell, i.e., they do not exhibit high coupling efficiency for collecting the light. Radiation upon entry into the flow cell through window 16 will diffuse (fan-out). Diffusion of this light may result from many causes. Primarily, in this case, it results from a light source producing a beam of light having rays which originate from a plurality of point sources. And, all of these point sources are not aligned so that when the rays of light from each point source enter the inlet window 16, the rays are not exactly aligned with the length of the flow cell and the outlet window 18. Additionally, other forms of diffusion also occur; these forms include: (1) refraction of light as it crosses material boundaries, (2) scattering of light due to particles along the path length and (3) the tendency of light to spread out normal to its path of movement (beam spreading).
If the flow cell is narrow, as shown in FIGS. 1A and 1B, some light, due to fan-out (diffusion), may be absorbed in the walls or blocked by the walls of the flow cell. If the flow cell is wide, as shown in FIG. 1C, only a limited portion of the light upon entry into the inlet window 16 will be directed at window 18. If large amounts of optical energy are lost within the flow cell, then the detector will have a reduced ability to detect weak, e.g., strongly absorbed, illumination at the radiation detector. The weakest absorption signal that can be detected is limited by the "dark" current (noise) produced by a photodetector or photodetector array. As the illumination becomes weaker, the signal to noise ratio is reduced until a point is reached where it is not possible to distinguish between the signal produced by the weak illumination and the noise.
The signal to noise ratio between the detected illumination and the dark current may be increased by increasing the magnitude of the optical energy entering the sample cell. However, increasing the amount of light transmitted into the flow cell could also result in increasing the temperature of the sample. Sample heating could cause chemical reactions or bubbles to come out of solution resulting in the sample no longer being representative of the material from which the sample was obtained.
The windows 16, 18 of FIGS. 1A, 1B, and 1C are also sources of inherent design limitations. They may be an integral part of the flow cell or they may be removable. In any case, they become dirty or scarred from the sample material. This reduces the performance of the absorption detector. The windows must then be cleaned or replaced. Whether the windows are an integral part of the flow cell or separately attached, the windows are difficult to inspect, clean or replace in typical detector-cell arrangements.
Safety concerns related to electrical components of the absorption detector also affect the utility of this type of detector. The cost to install this absorption detection system could be expensive due to the expenditures necessary to meet fire and building codes. These safety codes are necessary because: (1) the sample cell may contain hazardous and/or explosive materials when sampling, and/or (2) the detector's electrical circuitry, which powers the means for producing the radiation and the means for producing the absorption spectrum signal, could ignite hazardous or explosive vapors in the local area or vapors from the sample cell. For example, when the arrangements shown in FIGS. 1A, 1B and 1C are used for in-plant monitoring, the means for producing the radiation, the means for producing the absorption spectrum signal and the detector-cell are housed in separate compartments with the compartments being located adjacently to, and inline with, each other so that optical energy may be transmitted between the compartments. The need to locate these components near each other and within the line of sight of each other, yet separate them to meet safety codes substantially reduces the options available in locating an absorption detector within a processing environment.
A simple, compact, robust and inexpensive absorption detector is needed which features a detector-cell configuration that: (1) reduces bubble noise, (2) compensates for and utilizes the diffusion of radiation to maximize optical coupling, (3) allows easy access to the windows for inspection or replacement, (4) increases the sensitivity of the detector without increasing the dead zone of the sample cell and (5) provides for greater selectivity in locating absorption detector components to meet safety codes.